Following on from the post on Converting Text to Audio, there is a fantastic service at spokentext.net.
This service allows you to upload a file (be it .txt, .pdf, .doc or .html) and it converts the file to .mp3 for you. The voices are OK, but it's the small things that are the best part about the service. You are provided with an RSS feed, meaning that every document that you convert becomes part of a podcast. You can subscribe to this in iTunes for example, and that will sync to your iPod if you use one.
Also there is a Firefox extension for it. Install the extension and you get a tool bar.
Select text in the web page that you are viewing, click the 'Record Selected Text' link, and it will add an audio version of the text to your podcast feed.
Using the Firefox extension and the RSS feed make this a very easy way of keeping consuming textual materials on your portable media player while you are on the move. It is free at the moment, but if you'd like to help the development of the service they are asking for donations to help pay for better voices.